This invention relates to methods and portable apparatus for testing engine exhaust, particularly the exhaust from large industrial engines.
Large, industrial engines are used for a variety of purposes, including: to generate electrical power; to drive pumps; and to drive compressors for the compression of natural gas in pipelines. In use, these engines emit a variety of gases, including carbon monoxide (xe2x80x9cCOxe2x80x9d), carbon dioxide (xe2x80x9cCO2xe2x80x9d) and nitrogen/oxygen compounds (xe2x80x9cNOxe2x80x9d and xe2x80x9cNO2xe2x80x9d) Concern about the environmental effect of the exhaust from these engines has resulted in widespread regulation of the operation of these engines, and particularly regulation of exhaust emissions. In many countries, these engines may not be operated without a permit granted by the relevant regulatory body.
Typically, such permits set out maximum emission limits for specified gases. The permit for a particular engine may merely set out a maximum emission rate for each specified gas or it may specify a maximum emission rate for each specified gas at a specified engine load. To ensure that the engine complies with the permitted emission rate, such permits also typically require that the engine emissions be monitored using a specified testing protocol. The permit may require that the emissions be monitored continuously, but more commonly, such permits require that the engine be tested periodically, such as every year.
The test protocols for periodic engine emission testing typically require that a series of tests of set duration be conducted. As well, the test protocols typically specify pre-test and post-test calibration procedures for the gas sensors used to measure the concentration of the test gases. Typically, when an industrial engine is tested for compliance with the permitted emission rate, neither the emission rates of the test gases nor the engine load can be easily measured directly. Rather, the test protocols provide for a variety of different measurements to be taken so as to enable the testers to estimate the emission rates of the test gases and the engine load.
It is difficult to measure the weight per unit time of a given regulated effluent gas (test gas) directly, so it is conventional to measure the concentration of the test gas in the exhaust and the volume of the exhaust gas and from those measurements compute the rate of emission of the test gas in pounds per hour (lbs/hr) or other designated units of measurement. In simple terms, the emission rate of a test gas is determined by: measuring the concentration of the test gas, typically in parts per million; determining or at least estimating the exhaust gas volumetric flow (that is, the rate of exhaust gas emission as indicated by a unit of volume over a unit of time); and using these two numbers to estimate the emission rate of the test gas.
It is, however, further difficult to accurately directly measure the volumetric flow of the hot, turbulent exhaust gas. Therefore, conventionally, the exhaust gas volumetric flow is also estimated. For an engine powered by natural gas, the exhaust gas volumetric flow can be estimated from: the volumetric flow of the fuel gas; a fuel factor constant; and the concentration of oxygen (O2) in the exhaust gas. The volumetric flow of the fuel gas can be measured directly with a flowmeter, but it must be corrected for temperature and pressure to be of use in estimating the exhaust gas volumetric flow. The fuel factor constant is determined from the concentrations of the constituent compounds of the fuel gas. In simple terms, the exhaust gas volumetric flow is estimated by determining the corrected volume of fuel gas and calculating, on the basis of the fuel gas composition, what the volume will be after combustion, with a correction for the concentration of O2 in the exhaust gas.
As well, using previously known procedures and conventional portable apparatus for engine emission testing, the engine load is usually estimated from the work done by whatever equipment the engine is driving. For example, if the engine is driving a compressor, the work done by the compressor may be determined by measuring the pressure and volumetric flow of gas upstream of the compressor, and the pressure of the gas downstream of the compressor. Such measurements can be used to determine the work done by the compressor, but, due to power losses in the compressor, and in the linkage between the engine and the compressor, they may not be an accurate indicator of the engine load. Depending on these power losses, the actual engine load may be up to 12% greater than the engine load estimated by this method, resulting in errors in the emission test results. While some tolerance for such errors can be taken into account when the regulatory authority sets emission standards, it would be preferable to obtain more accurate measurements of engine load.
The concentrations of the test gases can be measured directly with any of a variety of commercially available gas analyzers, including electrochemical, non-dispersive infrared and chemiluminescence gas analyzers. Typically, these gas analyzers contain sensors (also referred to in the trade as xe2x80x9ccellsxe2x80x9d) for measuring the concentration (in parts per million) of the gases specified in the engine permit (usually CO, CO2, NO and NO2) As well, the gas analyzers typically also measure the concentration of O2. In the known procedures for analyzing engine emissions, the O2 measurements are used as indicators of whether the engine is running in a rich or lean combustion state.
The sensors may be cross-sensitive in that their accuracy may be affected by the presence of non-target gases (referred to as xe2x80x9cinterfering gasesxe2x80x9d). Cross-sensitivity is also referred to as the interference response. A sensor""s cross-sensitivity to a particular interference gas is tested by exposing the sensor and a sensor targeted to the interference gas, to a test gas containing the interference gas but not containing the target gas of the sensor being tested for cross-sensitivity. For example, a NO2 sensor""s cross-sensitivity to NO would be tested for by exposing the NO2 sensor and a NO sensor to a test gas containing NO but not containing NO2. Any response by the NO2 sensor to the test gas would be due to cross-sensitivity. Cross-sensitivity may be quantified by comparing the interference response of the sensor being tested (the NO2 sensor in the example) with the response of the interference-gas-targeted sensor (the NO sensor in the example).
The measurements from the gas sensors may not be stable, in that they may have a tendency to drift over time when the sensor is exposed to a gas with a constant concentration of the relevant test gas. This quality of the sensors is referred to as stability or sensor drift, the two terms implying opposite characteristics. Sensor drift may be evaluated by exposing the sensor to a calibration gas and noting how the sensor measurements vary over time. The extent of sensor drift is often stated as the maximum absolute percentage deviation from an average measurement recorded shortly after the measured response time of the sensor.
Further, the accuracy of the measurements from a sensor may not be consistent over a range of concentrations, particularly when the sensor is subject to rapidly changing concentrations of the test gas. This quality of a sensor is referred to as degree of linearity of the sensor, or simply xe2x80x9clinearityxe2x80x9d. Linearity is tested by first exposing a sensor to at least two gases having different concentration of the test gas, one after the other, and observing the response of the sensor over time to the different concentrations of the test gas.
The test protocols typically require that the sensors be calibrated within a specified period before and after the relevant test. The test protocols typically require that the sensors be tested for calibration error and cross-sensitivity before and after each test run. The calibration error test results may be used to correct the sensor""s measurements, or if they fall outside of the required parameters, they may be cause to reject the results from the test run as unreliable, and possibly to indicate the need to replace the sensor.
The testing procedure typically involves transporting a gas analyzer to the engine location; connecting it to the exhaust stream; running the required tests and recording the test data; disconnecting the gas analyzer; removing it from the test location; and processing the data to generate the test results at some later date.
The conventional delay in processing the test data means that it is not known whether the engine has met the required emission standards until the testing is complete and the data can be processed, which typically does not occur until after the testing equipment has been removed from the engine site. If it turns out from the later data processing that an engine has failed a test, it is typically necessary to re-transport the testing apparatus to the test site and reinstall the testing equipment in order to rerun the test. In some cases, tuning the engine might make the difference between meeting the permit requirements and failing the test. However, various previously known testing procedures do not provide feedback of data on the engine emissions in real time, and therefore offer no guidance with respect to tuning the engine.
For an engine powered by natural gas, the data required to determine the emission rates of the test gases at a certain engine load include: the concentration of the test gases in the engine exhaust; the concentration of O2 in the exhaust; the fuel gas volumetric flow; the fuel gas temperature; the fuel gas pressure; and the engine load. The concentrations of the specified test gases are recorded electronically. However, with the known procedures for performing emission testing, the fuel gas volumetric flow, the fuel gas pressure; the fuel gas temperature, and the engine load are merely written down by the person conducting the test. Typically, this handwritten information is later manually entered into a computer database or spreadsheet for processing with other information recorded during the test. It is clear that errors can occur both at the initial note-taking and later when the information is subsequently entered into the computer.
What is needed is a portable engine emission analyzer that: produces engine emission information in real time; permits the generation of a test report immediately after an emission test is conducted; reduces the risk of operator error; and is used in combination with a more accurate source of engine load information.
In ordinary engineering parlance, and in this specification, xe2x80x9cin real timexe2x80x9d means data and actions on data occur or are available in real time, or are so time-correlated to the sequence of physical events to which the data relate so as to provide the same benefit, for all practical purposes, as if they had occurred or were available simultaneously with the physical events to which they relate.
The method of emissions testing and determination of the present invention differs from most conventional prior methods in that it comprises a method of testing an engine and analyzing the exhaust emissions of the engine and making the necessary computations to determine the emission rate of a specified test gas or test gases in real time. Most conventional prior methods are incapable of providing real-time results but instead require a delay between the measurement stage of the method and at least part of the computation stage of the method.
The preferred method of analyzing the exhaust emissions of gas-fueled engines in real time according to the invention is capable of providing a relatively accurate determination of the emission rate for the test gas (to the extent that the sensors used are reliable and that the test equipment is accurately calibrated).
In one aspect of the invention, suitable for use for determining the amount of a specified test gas in the exhaust of an engine powered by natural gas fuel or other gaseous fuel, the method includes the steps of:
(a) measuring the flowrate, temperature and pressure of the fuel gas;
(b) measuring the relative concentration of the test gas in the engine exhaust;
(c) computing a volumetric flowrate of the exhaust gas from the measurement data representing the flowrate, temperature and pressure of the fuel gas;
(d) computing an emission rate (conventionally expressed in the units lbs/hr) of the test gas from the exhaust gas volumetric flowrate data and the data representing the relative concentration of the test gas;
(e) determining or measuring the engine load at which steps (a) and (b) occur, and computing an emission rate per engine load (conventionally expressed in the units lbs/BHP-hr) from the computed emission rate and engine load;
(f) optionally, recording or displaying the calculated test gas emission rate; and
(g) optionally performing further calculations and tabulations of the gas emission rate data, e.g. comparing the calculated test gas emission rate to the maximum permitted emission rate and, if the calculated emission rate is greater than the permitted emission rate, displaying a warning or alarm or record or display of this result.
In the foregoing summary, and in this specification generally, the step of xe2x80x9cmeasuringxe2x80x9d may be a composite step involving measurement of one or more given parameters and then performing a calculation on it to derive an estimated value for the parameter whose value is sought. In other words, measured values include estimated values where direct measurement of a parameter is difficult. Further, in the above summary, and in this specification generally, the step of xe2x80x9ccomputingxe2x80x9d or xe2x80x9ccalculatingxe2x80x9d includes providing as an interim or final output the results of the computation in digital data format. Further, the step of xe2x80x9cmeasuringxe2x80x9d is to be taken as including, as necessary, the conversion of any analog measurement data to digital format. All of the foregoing computation steps may be performed in real time by a programmed computer and may be repeated a preselected number of times at preselected time intervals.
As discussed above, the method normally includes the step of determining the engine load (typically expressed in BHP) so that an emission rate per engine load (typically lbs/BHP-hr) may be calculated and recorded or displayed as required. However, in some cases this step may not be necessary. For example, if the maximum emission levels in a permit are not in terms of emission rate per engine load, there may be no need to determine the engine load or compute the emission rate per engine load.
Preferably, the step of determining the engine load comprises correlating in real time engine data, including intake manifold temperature and pressure, and the engine RPM (usually manually entered as an input into the programmed computer) with a load curve for the engine. A load curve appropriate to a particular engine is typically prepared by, and obtained from, the manufacturer of the engine. A load curve is typically specific to a particular model, but a load curve may instead be specific to a particular conformation (e.g. turbo-charged or naturally aspirated) of a particular model, or to a particular conformation of a particular model under particular operating conditions (e.g. intercooler water temperature or manifold temperature). Correlating the engine data to the load curve involves: selecting the appropriate load curve, which entails comparing some of the known engine data with the load curve selection criteria; and then using the load curve data and additional engine data to calculate the engine load. Preferably, discrete values not found on the engine manufacturers load curve are calculated using Newton""s Method of Interpolation. Preferably the data from each load curve is incorporated into a computer routine (sometimes referred to in the trade as a xe2x80x9cfunctionxe2x80x9d) along with Newton""s Method of Interpolation, such that the programmed computer may xe2x80x9ccall the functionxe2x80x9d, that is, provide a particular routine with values for the required variables and instruct the routine to calculate the engine load.
Alternatively, the engine load may be approximated by correlating the engine RPM with the engine manufacturer""s RPM-engine load specifications(preferably stored in the computer-readable database).
In another aspect of the invention, an engine emission analyzer comprises a programmed computer connected to: a data collection buffer, a computer-readable database and a display device. The data collection buffer is configured to connect to, and receive data from: a gas analyzer for sensing the relative concentration of the specified test gas or gases and O2; an intake manifold temperature sensor; an intake manifold pressure sensor; a fuel gas flowmeter; a fuel gas pressure sensor; and a fuel gas temperature sensor. The data buffer is configured to: accept the sensed data (some in analog and some in digital form), digitize the analog data, organize the data into batches that can be recognized by the programmed computer, and send the batches to the programmed computer. The computer-readable database is for storing data used in performing the engine emission analysis, including: the specifications of the engine being tested; the maximum emission limits for each of the test gases (pollutants) specified in the relevant permit; the testing parameters; and various calculation factors. The display device is preferably a display screen, but may be a printer or any other suitable means for indicating, recording or storing test results for the benefit of the user. The programmed computer is programmed to receive batches of data from the data buffer; to call up and receive data from the database as required; to perform the engine emission analysis calculations; and to send data representing computed emission rates and other relevant data to the display device.
Preferably, the data collection buffer is configured to connect to, and receive data from, a second gas analyzer and two exhaust temperature sensors, so that the apparatus may also be used to test the effectiveness of in-line catalyst elements (catalytic converter) for treating exhaust. In such use, the data collection buffer is connected to an upstream gas analyzer that draws exhaust gas from upstream of the catalytic converter, an upstream temperature sensor that senses the exhaust temperature upstream of the catalytic converter; a downstream gas analyzer that draws in exhaust gas from downstream of the catalytic converter; and a downstream temperature sensor that senses the exhaust temperature downstream of the catalytic converter. The data collection buffer digitizes this data, organizes it into batches that can be recognized by the programmed computer, and sends the batches to the programmed computer. The programmed computer calculates the differences between the upstream and downstream levels of the relevant test gas or gases; calculates the difference between the upstream temperature and the temperature necessary to stimulate the desired chemical reaction between the catalyst and the exhaust gas; calculates the difference between the upstream and downstream temperature of the exhaust gas (an indicator of the extent to which the desired catalyzed reaction, which is exothermic, is occurring); and sends the calculated results to the display device in real time.
Preferably, the programmed computer is programmed to calculate, in real time, and send to the display device, brake-specific fuel consumption (BSFC) for use as a guide to engine tuning. The BSFC, typically expressed as rate of fuel consumption per engine load, is an indication of engine efficiency, with a lower BSFC indicating greater efficiency than a higher BSFC. In use for tuning an engine, the programmed computer calculates BSFC from the measurements of intake manifold pressure and temperature, and fuel-gas flow, pressure and temperature, and sends the digitized result to the display device or other suitable recording device.
The various features of novelty that characterize the invention are pointed out with more particularity in the appended claims. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be made to the accompanying drawings and descriptive matter in which there are illustrated and described preferred embodiments of the invention.